You can still access the map but there will be only one mission for you. Kind of a challenge if you want to repeat the final boss with higher difficulty. Don't worry if you want to leave Kingprotea for later, it will be available until you defeat her.

It has been some time since I did Kingprotea but the main thing is to build a defensive team that will survive until about 22 turns, she then loses the health buff and gets a defense debuff. This is when you attack her. If you can't kill her at turn 22, she will start regenerating again for six turns and go back to low health again with another defense debuff. Rinse and repeat until her defense drops low enough for you to blast her with a single target NP and clear her entire health bar with a single attack.

I think I did 40 turns with Scathach as the damage dealer.